1. Field of the Invention
The present invention relates to an image processing type of measuring device used for measuring the size or form of an object to be measured from an image of the measured object obtained with an optical system, a lighting system or the measuring device, a method of controlling the lighting system, a control program for the lighting system, and a recording medium with the lighting system control program recorded therein. More specifically the present invention relates to a lighting system or lighting control for an image processing type of measuring device capable of stabilizing luminous intensity or chromaticity of light irradiated to an object to be measured.
2. Description of the Related Art
There has been used an image processing type of measuring device capable of optically magnifying a measured portion of an object to be measured with a magnifying optical system and measuring the size or form of the objected to be measured from the magnified image. The image processing type of measuring device as described above includes, for instance, a microscope, a projector, and a three-dimensional image measuring device. In the image processing type of measuring device, lighting to an object to be measured plays an extremely important role for obtaining an image of the measured object.
As a lighting system for the image processing type of measuring device as described above, there have been known, in addition to the vertical incident-light system in which light is irradiated from a point substantially just above an object to be measured to the measured object, the diagonal incident-light system in which light is irradiated to an object to be measured from a point positioned in a direction inclined at a prespecified angle against an optical axis of the image pick-up optical system, the translucent lighting system in which light for illumination is irradiated from a point substantially just below an object to be measured to the measured object.
Recently a LED (Light Emitting Diode) is used as a luminescent light source for the lighting systems as described above. In the LEDs, generally the light intensity and chromaticity largely vary due to such factors as errors generated during the production process at the manufacture. To overcome this problem, LEDs are ranked to several types according to the light intensity and chromaticity for sale on the market (See FIG. 11).
The purchasers are required, however, to specify the ranks of the LEDs before purchase, which causes increase of the cost, and even if a purchaser buys LEDs specifying the rank before purchase, there still remains the variance of product quality within the rank, which may quite negatively affect the image processing type of measuring device. For instance, if light intensity varies from a piece of LED to another piece of LED, reliable compatibility of the part program among image processing type of measuring devices is lost.
Further, when red, green, and blue lights emitted from the respective LEDs are mixed and the synthesized light is irradiated to an object to be measured, if luminous intensity of the synthesized light is changed, sometimes the same chromaticity can not be reproduced.
When it is required to control light intensity of an LED, generally intensity of the emitted light, namely luminous intensity of lighting is controlled by changing an impressed current (forward current) through the LED chip. However, as the light intensity characteristics of each LED is non-linear against changes of the current (See FIG. 12), for instance, when current values for LEDs emitting lights with different colors respectively are changed at an equal pitch, a percentage of each color in the synthesized light varies, so that the specific chromaticity of the synthesized light can not be reproduced stably. This change in the chromaticity is magnified by aberration of the optical element, which in turn degrades the accuracy in measurement.
To describe in further detail, the image processing type of measuring device comprises, for instance, a lighting system for irradiating light to an object to be measured, a light-receiving sensor for receiving the reflected light from the measured object, and an image processor for obtaining a form of the measured object from an image received by the light-receiving sensor. The lighting system comprises, for instance, a plurality of luminescent light sources (LEDs) capable of emitting lights with different colors respectively, an impressed current control section for controlling an impressed current to each of the luminescent light sources for light emission, and an instruction value input device which can be operated from the outside for setting therein the luminous intensity of lighting as an instruction value.
The chromaticity coordinates (x, y) for light emission is previously specified for each LED as a luminescent light source for light with a specific color, and therefore for synthesizing light with a specified synthesized color, for instance, white color light with the CIE 1931 chromaticity coordinates (0.3, 0.3), the mixing ratio of lights emitted from the LEDs for element colors (rR, rG, rB) is computed. Then the intensity of light emitted from each LED corresponding to the luminous intensity L instructed by the instruction value input device is decided as shown below respectively. In the following equations, LR indicates the luminous intensity of the LED emitting light with red, LG indicates the luminous intensity of the LED emitting, light with green, and LB indicates the luminous intensity of the LED emitting light with blue.LR=rRLLG=rGLLB=rBL
Then values for impressed currents required to make the LEDs emit lights according to the specified luminous intensities of LR, LG, and LB respectively are computed. The impressed current control section impresses currents to the LEDs according to the instruction values.
In the configuration as described above, a user instructs the luminous intensity L for lighting with the instruction value input device. Then the impressed current control section allows impression of the specified impressed currents to the luminescent light sources so that synthesized light having a specified chromaticity such as, for instance, a white color will be synthesized and irradiated according to the instructed luminous intensity. When the specified currents are impressed to the LEDs, the luminescent light sources for light emission emit lights according to the specified luminous intensities (LR, LG, LB), and white light is irradiated to an object to be measured according to the instructed luminous intensity. The reflected light from the measured object is received by the light-receiving sensor, and such parameters as the form or size of the measured objected are measured by, for instance, detecting edges of the measured object from the received image.
In measurement of an image, the luminous intensity and chromaticity of light irradiated to an object to be measured is very important For instance, when the actual luminous intensity for lighting is lower as compared to the specified one, the amount of light is insufficient and an image can not be obtained, or edge detection can not be performed. When the actual luminous intensity for lighting is higher as compared to the specified one, the light is saturated, so that the image is blur (or partially lacked) and edge detection can not be performed also in this case.
Further, when chromaticity of lighting includes aberration caused by the white light, as an image of a colored objected to be measured can not be picked up accurately, edge detection can not be performed also in this case. Namely, control of impressed currents to a red LED, a green LED, and a blue LED to accurately control the luminous intensity and chromaticity for synthesized lighting is extremely important for precise measurement.
As each LED has the individual difference (variance), also the luminous intensity of each discrete diode disadvantageously varies even if the current having the same rated value is impressed thereto. FIG. 26A, FIG. 26B and FIG. 27 show the relations between impressed current values and the luminous intensities when a red LED, a green LED, and a blue LED are made to emit lights with respective colors in each lighting system. For instance, in the luminous intensity characteristics of the red LED shown in FIG. 26A, when an impressed current becomes larger, also the luminous intensity becomes higher in all devices, but as understood from the comparison between the device 1 and the device 4, a difference in the luminous intensity substantially varies in a device to which a current with a higher value is impressed. This is true also for the green LED shown in FIG. 26B and the blue LED shown in FIG. 27. Namely an impressed current value required to make each LED emit light at a specified luminous intensity value from an LED to an LED, so that, even if the luminous intensity is instructed, a value of an impressed current from for each discrete LED can not easily be decided.
When a current is impressed according to an instructed value for the impressed current, the luminous intensity of each LED may be off from the desired value, which may in turn result in intensity of synthesized light off from the instructed value. If luminous intensity of each LED is off from the instructed one, then also the chromaticity of the synthesized light is disadvantageously off from the desired one.
Further the chromaticity of light emitted from each LED includes the individual difference, and in addition the chromaticity changes in correlation to a change in intensity of the emitted light. FIG. 28, FIG. 29A, FIG. 29B, FIG. 30A, and FIG. 30B show the correlations between the luminous intensity and the chromaticity when a red LED, a green LED, and a blue LED are made to emit light respectively.
For instance, the x coordinate for the chromaticity of a blue LED in each lighting system is shown in FIG. 30A. This figure shows how a chromaticity coordinates changes in response to changes in luminous intensity. Further this figure shows that the chromaticity is different in each lighting system. It is understood from FIG. 28, FIG. 29A, and FIG. 29B that the same is true for a red LED and a green LED.
In other words, as the chromaticity of light emitted from each LED is different, if a mixing ratio of each light for generating synthesizing light is fixed according to a characteristic, the chromaticity of synthesized light is disadvantageously different from the instructed one.
When the intensity of light emitted from each LED is adjusted, taking into considerations the difference of luminous intensity characteristics of each discrete LED, for changing a value of the current impressed to the LED to achieve the desired luminous intensity with synthesized light, it is possible to achieve the accurate luminous intensity. In this case, however, if the luminous intensity is changed, also the chromaticity changes, and therefore there occurs the problem that the chromaticity of synthesized light is off from the desired value.
On the contrary, even if a color mixing ratio of lights emitted from the LEDs is decided by paying attention to the chromaticity for realizing the desired chromaticity for lighting with synthesized light, the chromaticity of light emitted from of each diode is different, and further the chromaticity of emitted light changes in correlation to the luminous intensity, so that the chromaticity of a LED can not be decided unitarily and a mixing ratio for achieving the desired chromaticity can not simply be decided.